Multicasting is a service that permits a source to send a message to a plurality of specific recipients. The notion multicasting typically involves that only one copy of the message will pass over any link in a network and copies of the message will be made only where paths diverge. From a network perspective, multicasting dramatically reduces overall bandwidth consumption, since data is replicated in the network at appropriate points.
With Internet Protocol IP multicast, receivers do not need to know who or where the senders are and senders do not need to know who the receivers are. Neither senders nor receivers need to care about the network topology as the network optimises delivery. The distribution of information via IP multicast is performed on the base of a hierarchical connection of the hosts, like for example a tree. Several algorithms have been proposed for building multicast distribution trees, like for example spanning trees, shared-trees, source-based trees, core-based trees. The descriptions of the corresponding algorithms can be found in “IP telephony: Packet-based multimedia communications systems” O. Hersent, D. Gurle, D. Petit, Addison-Wesley, Harlow, 2000. After the establishment of the distribution tree, the distribution of the information is done by the IP multicast routing protocols. The detailed description of the corresponding IP multicast routing protocols can be also found in the above-mentioned document.
An advantage of the IP multicast is the support of the heterogeneous receivers. Sending the different media to different multicast groups allows receivers to decide which media to receive. Similarly, if a sender layers its video or audio stream, different receivers can choose to receive different amounts of traffic and hence different qualities. To do this the sender must code the video as a base layer it means with the lowest quality that might be acceptable and a number of enhancement layers. Each enhancement layer adds more quality at the expense of more bandwidth. For video transmission, these additional layers might increase the frame rate or increase the spatial resolution of the images or both. Each layer is sent to a different multicast group and receivers can individually decide to how many layers they wish to subscribe.
Local area networks have supported multicasting for many years. For typical CSMA (Carrier Sense Multiplex Access) networks where nodes share a common communication medium, multicasting is easy to support. Ethernet for instance supports unicast addresses, multicasting addresses and the network broadcast address. A specially addressed packet can be read off the communication medium by multiple hosts.
Extending multicasting capabilities to inter-networks however led to the introduction of a router at the edge of a network in order to figure out dynamically how to forward the datagram, whereby a datagram denotes a data packet which is established on the IP layer. The way of forwarding is delivered from the address included in the header of the datagram and from the routing table, which is administrated in the router. There are a few possibilities of doing the multicast addressing. The normal way of doing multicast addressing is by using dedicated multicast addresses. These addresses denote groups rather than individual recipients (as is the case with normal IP addresses). The task of the router in this case is to encode the destination addresses and to route it according to the information from the routing table.
Multicast is a receiver-based concept, it means the receivers join a particular multicast session group by informing a corresponding multicast router and traffic is delivered to all members of that group by the network infrastructure. The transmission can be performed either from one user to many users, the so-called one-to-many multicast or many users send information to many receivers, the so-called many-to-many transmission. Within the IP multicast, the membership of a multicast session group is dynamic it means that the hosts may join and leave groups at any time.
The technical background for multicasting is covered in the following:
IP Telephony: Packet-based multimedia communications systems. Hersent, O., Gurle, D., Petit, D., Addison-Wesley, Harlow, 2000.
According to known GPRS and UMTS networks, the mobile terminated down-link traffic is routed using identities which are designed to only handle point to point traffic. In order to reach multiple end users, multiple unicast messages have to be sent.
The technical background about GPRS in GSM and UMTS is covered in the following:
3GPP TS 03.60 V7.5.0 (2001-01) 3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects,
Digital cellular Telecommunications System (Phase 2+),
General Packet Radio Service (GPRS),
Service Description, Stage 2 (Release 1998).
3GPP TS 23.060 V3.6.0 (2001-01)
3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects,
General Packet Radio Service (GPRS),
Service Description, Stage 2 (Release 1999).
There is currently no mechanism available for point to multipoint traffic for wireless terminals when IP addressing is not used, c.f. IP multicast.